Digital still cameras and digital video cameras for capturing color images, using a single plate photosensitive sensor (image pickup device) and a color filter array, have been widespread. A color filter array is formed such that tiny filters each transmitting only a particular color such as R, G, or B are arranged cyclically. As a typical filter arrangement method adopted in a color filter array, Bayer pattern has been known.
In the Bayer pattern, filters of respective colors, namely R (red), G (green), and B (blue), are arranged in a mosaic for each logical pixel block (hereinafter simply referred to as a block) of 2*2 pixels. Among them, G filters are arranged over 2 pixels diagonally positioned in a block of 2*2 pixels. This means that G filters are arranged over pixels at top left and bottom right positions, or top right and bottom left positions, of a 2*2 pixel block, and an R filter and a B filter are arranged over the remaining pixels. Consequently, signal values of R, G0, B, and G1 are obtained for each block of 2*2 pixels. Among them, G0 and G1 are two signals of G components diagonally positioned in the block.
Meanwhile, as the quantity of data of image signals captured by a digital camera is very large in the state of current signals, the quantity of data is reduced by image compression encoding processing in many cases.
Patent Document 2 discloses an example of an encoding method for a Bayer pattern image. In the method disclosed in Patent Document 2, an average value Gi of four neighboring G pixels, on the top, bottom, left, and right, is subtracted from values of R and B pixels of a Bayer pattern to thereby form a Bayer pattern with (R-Gi), Gi, (B-Gi). Then, based on the (R-Gi), Gi, (B-Gi) Bayer image, interpolation processing is performed on each space coordinate to thereby calculate an RGB pixel value in each space coordinate. Then, the RGB pixel value, calculated in this manner, is transformed to YCbCr, and is encoded using a known method such as JPEG.
Patent Documents 3, 4, and 5 disclose other exemplary encoding methods for a Bayer pattern image. In the methods disclosed therein, color transform is performed directly on an image signal of a Bayer pattern to generate a two-dimensional signal of 4 components, and encoding is performed independently on an image of these 4 components. This means that operation of color transform is performed among R, G0, B, and G1 pixels in a block of 2*2 pixels, and each transformed component is input to a compressor. In Patent Document 3, regarding R, B, and G1, a difference between G0 is encoded, while G0 component is encoded directly. Further, in Patent Document 4, R, G0, B, and G1 are transformed to Y, Dg, Co, and Cg respectively, and in Patent Document 5, R, G0, B, and G1 are transformed to Y, Cb, Cr, and Cg respectively, and then, they are encoded.
Patent Document 1: JP 2001-285643 A
Patent Document 2: JP 2008-311874 A
Patent Document 3: JP 2005-286415 A
Patent Document 4: JP 2006-121669 A
Patent Document 5: JP 4436733 B